Various light modulator structures are well known in the art. Such structures includes liquid crystal displays (LCDs), light emitting diodes (LEDs), and micro-electronic mirror systems (MEMS). LCDs may be reflective or transmissive. Crystalline silicon may be used to manufacture liquid crystal on silicon (LCOS) displays.
With reference to FIG. 1, a conventional display system 10 includes a spatial light modulator (SLM) 12 connected to a drive circuit 14. The drive circuit 14 provides a drive signal 16 to the SLM 12.
With reference to FIG. 2, in a liquid crystal display system 20, liquid crystal material 22 is positioned between two electrodes 23 and 24. The liquid crystal material includes crystals 25 which are affected by the voltage applied across the two electrodes 23 and 24. One electrode 23 is grounded and the other electrode 24 is connected to a drive signal. For example, the drive signal may be a DC voltage signal. In the example illustrated in FIG. 2, when a voltage of zero volts (0 V) is applied to the electrode 24 the crystals 25 lie in a plane approximately parallel to the plane of the electrodes 23 and 24.
With reference to FIG. 3, changing the state of the voltage applied to the electrode 24 causes a corresponding change to the state of the crystals 25. In the example illustrated in FIG. 3, when a voltage of three volts (3 V) is applied to the electrode 24 the crystals 25 change their orientation to lie in a plane approximately perpendicular to the plane of the electrodes 23 and 24. Changing the orientation of the crystals 25 changes the polarization properties of the liquid crystal material 22.
With reference to FIG. 4, a drive signal 40 has a voltage of 0 V at time T0, changing to Von at time T1 and back to 0 V at time T2. When the drive signal changes voltage levels, the liquid crystal material 22 transitions between respective parallel and perpendicular orientations of the crystals 25. For example, one orientation corresponds to an ON state for a pixel element (e.g. a dark spot on the LCD) and the other orientation corresponds to an OFF state for the pixel element (e.g. a light spot on the LCD).
With reference to FIGS. 5-6, for an LCD system 50 the change from one orientation to another in one direction is relatively fast (see FIG. 5) while the change in the other direction is much slower (see FIG. 6). The relatively slower transition is limited by the relaxation properties of the liquid crystal material. The response time is related to the fluid dynamics. MEMS systems have similar mechanical properties where one orientation of the reflective element is influenced by an applied signal and the other orientation is dependent on mechanical restoring forces.
An important performance aspect of an SLM display system is the response time of the SLM. With reference to FIG. 7, a drive signal V is represented by the dashed line and the response time of the SLM is represented by the solid line. The horizontal axis T corresponds to time and the vertical axis A corresponds to normalized amplitudes of the drive signal and the ON state of the pixel. When a drive signal V is applied, the response time of the SLM under the influence of the applied signal (e.g. 3 V) is very fast, as represented by the steep ramp R in the graph. When the applied signal is removed (e.g. 0 V), the SLM relies on natural restoring forces to return the pixels to their original state. This transition is relatively slower, as represented by the curve C in the graph. LCDs, MEMS, and other conventional display systems all may have a response graph similar to the graph of FIG. 7.